With the recent advent and development of the exchange of information by way of digital messages, reliable and high-performance transmission of digital data has become the economic stake.
Among the transmission modes used, digital packet transmission occupies a pre-eminent place, by reason of the flexibility and of the reliability of the protocols for transmission of these data.
However, the development of transmissions at very high throughput on radio frequency channels, featuring characteristics of frequency selectivity which are variable over time, makes it necessary to submit the digital data constituting these messages and the medium for this information, to a process of protection by specific coding. These protection processes are for the purpose of introducing into the digital data a certain amount of redundancy, which, in the presence of degradation of these data due to the transmission, makes it possible, under certain conditions, to reconstitute the original signal. By way of non−Limiting example, mention may be made of the protection of digital data by a convolutional code of efficiency R=k/n where the efficiency k/n is representative of the redundancy introduced, and the ATM (Asynchronous Transmission Mode) transmission on a radio link, with granularity at the level of the ATM cells. It will be recalled that the notion of granularity implies the possibility of transmission of each ATM cell in isolation, without interlacing of the cells.
By reason of the physical nature of the radio frequency channel, the transmission of digital data takes place in the presence of multiple propagation paths. The reception and the decoding of these messages consequently require equalization of the received signal. The complexity of the equalization processing operations increases very rapidly with the dispersion in the delays, and, likewise, with the throughput of the symbols transmitted. This is particularly the case in the urban environment, upon transmission of mobile-telephony messages and signals.
The protection introduced by coding of these digital data transmitted makes it possible partially to remedy the errors introduced. However, the equalization made necessary by interference on the symbols, due to the multiple paths, is never perfect and it is then necessary, as far as possible, to disperse the error bursts at the output of the equalizer, so that the channel decoder can function on a signal with a substantially constant, further reduced error density.
Such a method of operating can be obtained by a process of interlacing of the coded data, which prevents the notion of granularity, and which, moreover, introduces a delay which it is necessarily sought to minimize, which, a priori, appears contradictory.
Furthermore, for high throughput, the size of the interlacing becomes prohibitive.
Moreover, the presence of a channel decoder downstream of the equalization device, the equalizing/decoding sequencing being made necessary by the coding/transmission/multiple-path sequencing on transmission, encourages the use of an equalization process giving flexible equalized symbols, that is to say symbols the firm value of which is accompanied by a likelihood probability value, which further increases the complexity of the equalization and of the overall processing.